The present invention relates generally to electroconductive molding compositions which are especially useful in fabricating shielding for sophisticated electronic equipment, power tools, automotive ignition systems, household appliances like microwave ovens, or for that matter, any component(s) or equipment which produces stray electromagnetic emissions and have the potential of damaging or adversely affecting the performance of other equipment or components. More particularly, the immediate invention relates to electrically conductive bulk, granular or nodular molding compounds comprising a multi-component filler system having at least two members from the group consisting of particulate carbon black, graphite and metal particles, the combination providing a spectrum of shielding to electromagnetic interference (EMI) which said individual fillers are unable to achieve without substantial reduction in mechanical properties and/or processing characteristics.
Heretofore, various methods have been used to shield electronic equipment. Metallic boxes and cans fabricated from steel, copper, aluminum, etc., were used by surrounding high EMI emitters for shielding. However, because shields fabricated from metal were cumbersome, heavy and costly the electronics industry has resorted to metallized plating on plastics. But, the results obtained with metallic coatings were not always satisfactory. In addition to being relatively non-economic, once such metallic coatings were scratched through they would lose part of their shielding efficiency. Unless such conductive coatings are continuous and free of voids, electromagnetic waves will be free to pass through. Frequently, it was difficult to obtain a dependable, 100% effective coating which was also resistant to peeling.
Further efforts by the electronics industry to develop more dependable light-weight materials for EMI shielding has led to a third approach, namely electrically conductive composites consisting of a variety of polymers loaded with conductive fillers and/or reinforcements. It was anticipated that intricate shapes could be molded from the composite materials by conventional means, yielding a finished part that promised to be more economic and dependable than metal or metal-coated plastics.
One of the most common conductive fillers used in the composite approach has been carbon black. The major advantage of resinous composites containing this filler has been that it flows readily in molding processes, and therefore, provides a high degree of design flexibility. However, in order to obtain nominally acceptable shielding efficiencies to EMI emissions loadings of carbon black in molding compounds should be above 15% by weight. A similar requirement also exists in the case of graphite powder. In each instance, the loading levels required of powdered and spherical-shaped conductive fillers has been too high to achieve acceptable conductivity without reducing the impact resistance and other mechanical properties needed for EMI shields and housings. Carbon/graphite fibers also provide acceptable EMI shielding efficiencies, but because of very high cost they are not viewed as acceptable alternatives to the more economic powders and nodules.
Metal particles of various sizes and shapes such as aluminum, copper, nickel, zinc, etc., have also been used in molding compositions as conductive fillers. The principle factor influencing the performance of metal-filled composites is the aspect ratio of the particles. For example, spherical-shaped particles must be loaded to a 38% by volume concentration to achieve electrical conductivity. However, this frequently leads to poor mechanical properties and poor cost effectiveness. In contrast, fibrous metal particles are able to impart electrical conductivity to composites with as little as 7% by-volume metal. However, high aspect ratio fibers are difficult to process in that they become entangled and agglomerate producing a poorly dispersed mixture.
A search of the literature, including patent publications relating to electrically conductive composites, uncovered the following: U.S. Pat. Nos. 1,556,990; 4,124,747 and 4,197,218. U.S. Pat. No. 1,556,990 relates to electrical brushes and contacts for machinery comprised of graphite, pulverized metals, carbon, coke or lamp black in a thermosetting phenolic resin binder. The compositions have a loading from 75-99% filler and up to 25% resin binder. Compositions with such high ratios of filler to binder will have downgraded mechanical properties. Carbon black is blended with thermoplastic polymer in U.S. Pat. No. 4,124,747. At low carbon black loadings the conductive behavior is very erratic and difficult to predict whereas at high filler loadings a penalty is incurred in physical properties. U.S. Pat. No. 4,197,218 discloses conductive compositions comprising a non-conductive plastic material and a ferroalloy. Ferrophosphorous, for example, is a material having a high bulk density, and suggested loadings of up to 90% by weight are excessive for satisfactory molding compounds used in fabricating light-weight shielding devices. By comparison, lower more tolerable levels of such ferroalloys in bulk molding compounds provide shielding coefficients to EMI emissions which are considered too low.
In the technical literature, D. M. Bigg et al. reported in Industrial Research/Development, pp. 103-105, July 1979, the use of aluminum flake and fiber in molded composites wherein at page 104 a polyester composite filled with 20% aluminum fiber provided about 40 dB of attenuation up to a frequency of only 30 Mhz but dropped below 30 dB at 100 Mhz. Polyester composites filled with 30% aluminum flake provided 30 dB of attenuation up to 100 Mhz but dropped below 20 dB at about 500 Mhz; indicating the inability of particulate aluminum-filled composites of maintaining sufficiently high shielding efficiencies over the entire spectrum of frequencies ranging from 0.5 to 1000 Mhz.
Plastics Compounding, page 19 etc., January/February 1980, describes the use of various conductive fillers including carbon black, metallized glass fibers, carbon/graphite fibers and aluminum fibers, flakes and ribbon. Plastics Design Form, page 16 etc., March/April 1979, also discloses the use of conductive fillers in plastic housings for EMI shielding. However, both authors fail to disclose how one may achieve shielding efficiencies which are constantly high (at least 30-40 dB) and substantially linear over the spectrum of electromagnetic frequencies (0.5 to 1000 Mhz). Iron Age, pp. 59-61, Oct. 1, 1979, suggests aluminum fibers over flakes dispersed evenly throughout molded thermoplastic resins with loading levels of 20-30% by weight. Combinations of conductive fillers to achieve a specific level of range and shielding are not suggested. Accordingly, there is a need for more dependable, high performance molding compounds which will provide consistently high shielding efficiencies to electromagnetic emissions without forfeiting mechanical and/or processing capabilities.
It has now been discovered that molding compositions comprising a multi-component conductive filler system is capable of providing unexpected advantages in EMI shielding. The multi-component system is able to provide at least 20 dB, and more preferably, 30-70 dB attenuation of constant shielding effectiveness over the entire frequency spectrum of 0.5 to 1000 Mhz as demonstrated by substantially linear shielding efficiency/frequency curves (FIGS. 1 and 2). The improved shielding performance is accomplished without adversely affecting the processing capabilities of the molding compound or the mechanical properties of the molded composites.